Propane backfire?

[partsman01]

So, did not see this topic anywhere, my 1977 F-250 400 CI was done by a fellow that used to do propane conversions, it is a dual fuel system. No brain box or what have you, so he apparently set it up I think to run best on propane, as it is great on propane and will break the wheels loose easier than on gas, gasoline is not as smooth running. So I imagine it is set up as a compormise system, anyway I have had it where going up hills it has done what I call backfiring, so I was told to take the rotor under the distributor cap and cut one side off, as it is what I call a hammerhead shape and that would stop it, as I guess the thought is that the spark is jumping quicker or something, so I think they said to do it on the following edge. Anybody ever heard of this?

[cookie88]

I'm not sure I understand. Is the assertation here that making the rotor lighter will make it spin faster? I don't think that's going to happen since the rotor is driven off the cam.

If you want your spark happening earlier maybe you should advance the timing a little bit.

[partsman01]

I assume he was saying that it cuts down on the spark jumping too soon, or something like that.
Seems it would focus the spark at the correct point for sparking.

[cookie88]

So did you try it? Did it work?

[partsman01]

Have not tried it yet, thought I would ask here first.
Have only city drove the truck so far now, have not been out on holidays with it yet.
May not get to use it for the usual trips and that is the only time I noticed the problem, due I guess to the heat and the heavy load.

[gasman6674]

Back fire on LP is usally caused by a lean mixture, Look for an air leak cracked hose etc.
You might be able to richen it up under load but I will need to know what kind of system you have.